It is absolutely necessary to ensure that the breathing equipment used by a diver or a firefighter, for instance, is fully serviceable and faultless prior to entering non-breathable atmospheres, for instance when diving or when working in smoke-filled or toxic environment.
Among other things, it is necessary to check that the system gas-supply is completely full and, therewith, contains the amount of breathing gas that can be expected to be consumed, that the hoses leading to the breathing mask are tightly sealed, that is, will not leak to the surroundings and, therewith, reduce the amount of gas available for breathing, that gas is able to flow from the gas reservoir freely and without hinder and will arrive at the breathing mask in sufficient volumes, that is, that there is practically no resistance to the air flow and that the pressure prevailing in the breathing mask is higher than ambient pressure.
The gas reservoir carried by the person concerned will normally have the form of a gas cylinder that contains breathing gas at a pressure of normally 300 bars, when the cylinder is full. The breathing gas is normally air, although under special circumstances may often contain at least 20 percent by volume oxygen and an inert gas, most often nitrogen and perhaps also helium. In some cases, for instance, for diving to great depths, the breathing gas contains less than 20 percent oxygen by volume. Since the gas reservoir has a relatively small volume, it is important that the reservoir pressure is sufficiently high to supply the user with an anticipated maximum gas volume.
It is also important that the hoses or lines leading from the gas reservoir are tight and that the flow resistance presented thereby is sufficiently small for the gas reservoir to deliver to the user a quantity of gas that is large enough to satisfy the user's requirements, even in the case of an extreme need.
Another important safety problem concerns the gas pressure in the mask when the mask is in place. The mask pressure must be greater than the ambient pressure, so that non-breathable atmosphere, particularly toxic atmosphere, is unable to penetrate into the mask.